Fuel Cell Component With Coating Including Nanoparticles

ABSTRACT

A product comprising a fuel cell component comprising a substrate and a coating overlying the substrate, the coating comprising nanoparticles having sizes ranging from 2 to 100 nanometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/707,705, filed Aug. 12, 2005.

FIELD

The disclosure generally relates to fuel cell components with a coatingincluding nanoparticles thereon.

BACKGROUND

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. The automotiveindustry expends significant resources in the development of hydrogenfuel cells as a source of power for vehicles. Such vehicles would bemore efficient and generate fewer emissions than today's vehiclesemploying internal combustion engines.

A hydrogen fuel cell is an electro-chemical device that includes ananode and a cathode with an electrolyte therebetween. The anode receiveshydrogen-rich gas or pure hydrogen and the cathode receives oxygen orair. The hydrogen gas is dissociated in the anode to generate freeprotons and electrons. The protons pass through the electrolyte to thecathode. The protons react with the oxygen and the electrons in thecathode to generate water. The electrons from the anode cannot passthrough the electrolyte, and thus are directed through a load to performwork before being sent to the cathode. The work may be used to operate avehicle, for example.

Proton exchange membrane fuel cells (PEMFC) are popular for vehicleapplications. The PEMFC generally includes a solid-polymer-electrolyteproton-conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA). MEAs are relatively expensive to manufactureand require certain conditions for effective operation. These conditionsinclude proper water management and humidification, and control ofcatalyst poisoning constituents, such as carbon monoxide (CO).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For the automotive fuel cell stack mentionedabove, the stack may include about two hundred or more bipolar plates.The fuel cell stack receives a cathode reactant gas, typically a flow ofair forced through the stack by a compressor. Not all of the oxygen isconsumed by the stack and some of the air is output as a cathode exhaustgas that may include liquid water as a stack by-product. The fuel cellstack also receives an anode hydrogen reactant gas that flows into theanode side of the stack.

The fuel cell stack includes a series of flow field or bipolar platespositioned between the several MEAs in the stack. The bipolar platesinclude an anode side and a cathode side for adjacent fuel cells in thestack. Anode gas flow channels are provided on the anode side of thebipolar plates that allow the anode gas to flow to the anode side of theMEA. Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode gas to flow to the cathode side ofthe MEA. The bipolar plates may also include flow channels for a coolingfluid.

The bipolar plates are typically made of a conductive material, such asstainless steel, titanium, aluminum, polymeric carbon composites, etc.,so that they conduct the electricity generated by the fuel cells fromone cell to the next cell and out of the stack. Metal bipolar platestypically produce a natural oxide on their outer surface that makes themresistant to corrosion. However, this oxide layer is not conductive, andthus increases the internal resistance of the fuel cell, reducing itselectrical performance. Also, the oxide layer frequently makes theplates more hydrophobic.

US Patent Application Publication No. 2003/0228512, assigned to theassignee of this application, and the disclosure of which is hereinincorporated by reference, discloses a process for depositing aconductive outer layer on a flow field plate that prevents the platefrom oxidizing and increasing its ohmic contact. U.S. Pat. No.6,372,376, also assigned to the assignee of this application, disclosesdepositing an electrically conductive, oxidation resistant and acidresistant coating on a flow field plate. US Patent ApplicationPublication No. 2004/0091768, also assigned to the assignee of thisapplication, discloses depositing a graphite and carbon black coating ona flow field plate for making the flow field plate corrosion resistant,electrically conductive and thermally conductive.

As is well understood in the art, the membranes within a fuel cell needto have a certain relative humidity so that the ionic resistance acrossthe membrane is low enough to effectively conduct protons. Duringoperation of the fuel cell, moisture from the MEAs and externalhumidification may enter the anode and cathode flow channels. At lowcell power demands, typically below 0.2 A/cm², water accumulates withinthe flow channels because the flow rate of the reactant gas is too lowto force the water out of the channels. As the water accumulates, itforms droplets that continue to expand because of the hydrophobic natureof the plate material. The contact angle of the water droplets isgenerally about 90° in that the droplets form in the flow channelssubstantially perpendicular to the flow direction of the reactant gas.As the size of the droplets increases, the flow channel is closed off,and the reactant gas is diverted to other flow channels because thechannels flow in parallel between common inlet and outlet manifolds.Because the reactant gas may not flow through a channel that is blockedwith water, the reactant gas cannot force the water out of the channel.Those areas of the membrane that do not receive reactant gas as a resultof the channel being blocked will not generate electricity, thusresulting in a non-homogenous current distribution and reducing theoverall efficiency of the fuel cell. As more and more flow channels areblocked by water, the electricity produced by the fuel cell decreases,where a cell voltage potential less than 200 mV is considered a cellfailure. Because the fuel cells are electrically coupled in series, ifone of the fuel cells stops performing, the entire fuel cell stack maystop performing.

It is usually possible to purge the accumulated water in the flowchannels by periodically forcing the reactant gas through the flowchannels at a higher flow rate. However, on the cathode side, thisincreases the parasitic power applied to the air compressor, therebyreducing overall system efficiency. Moreover, there are many reasons notto use the hydrogen fuel as a purge gas, including reduced economy,reduced system efficiency and increased system complexity for treatingelevated concentrations of hydrogen in the exhaust gas stream.

Reducing accumulated water in the channels can also be accomplished byreducing inlet humidification. However, it is desirable to provide somerelative humidity in the anode and cathode reactant gases so that themembrane in the fuel cells remains hydrated. A dry inlet gas has adrying effect on the membrane that could increase the cell's ionicresistance, and limit the membrane's long-term durability.

It has been proposed by the present inventors to make bipolar plates fora fuel cell hydrophilic to improve channel water transport. Ahydrophilic plate causes water in the channels to spread along thesurface in a process termed spontaneous wetting. The resulting thin filmhas less of a tendency to alter the flow distribution along the array ofchannels connected to the common inlet and outlet headers. If the platematerial has sufficiently high surface energy, water transport throughthe diffusion media will contact the channel walls and then, bycapillary force, be transported into the bottom corners of the channelalong its length. The physical requirements to support spontaneouswetting in the corners of a flow channel are described by theConcus-Finn condition,

${{\beta + \frac{\alpha}{2}} < {90\underset{\_}{{^\circ}}}},$

where β is the static contact angle formed between a liquid surface andsolid surface, and α is the channel corner angle. For a rectangularchannel α/2=45°, which dictates that spontaneous wetting will occur whenthe static contact angle is less than 45°. For the roughly rectangularchannels used in current fuel cell stack designs with composite bipolarplates, this sets an approximate upper limit on the contact angle neededto realize the beneficial effects of hydrophilic plate surfaces onchannel water transport and low load stability.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a product comprising a fuelcell component comprising a substrate and a coating overlying thesubstrate, the coating comprising nanoparticles having sizes rangingfrom 2 to 100 nm.

Other embodiments of the present invention will become apparent from thedetailed description provided hereinafter. It should be understood thatthe detailed description and specific examples, while indicatingembodiments of the invention, are intended for purposes of illustrationonly and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a graph of the result of Fourier-transform infrared spectra ofa coating produced according to one embodiment of the invention.

FIG. 2 is a graph of the result of Fourier-transform infrared spectra ofa coating produced according to another embodiment of the invention.

FIG. 3 is a photo micrograph of a coating according to one embodiment ofthe invention;

FIG. 4 is a cross-sectional view of a fuel cell in a portion of a fuelcell stack that includes bipolar plates having a coating to make theplate hydrophilic, according to one embodiment of the invention;

FIG. 5 is a broken-away, cross-sectional view of a bipolar plate for afuel cell including a coating defined by islands separated by openareas, according to another embodiment of the invention;

FIG. 6 is a broken-away, cross-sectional view of a bipolar plate for afuel cell including a coating, where the coating has been removed at thelands between the flow channels in the plate, according to anotherembodiment of the invention;

FIG. 7 is a broken-away, cross-sectional view of a bipolar plate for afuel cell where a coating is deposited over another coating that is onthe bipolar plate according to one embodiment of the invention;

FIG. 8 illustrates one embodiment of the invention including a processincluding first selectively forming a mask over the lands of a bipolarplate and thereafter depositing a coating over the bipolar plateincluding the mask;

FIG. 9 illustrates one embodiment of the invention including a processwherein the mask over the lands is removed to leave the coating onlyoverlying the channel of the bipolar plate;

FIG. 10 illustrates one embodiment of the invention including a processincluding first depositing a coating including silicon over the bipolarplate, then selectively forming a mask over the channels of a bipolarplate, and thereafter the coating over the lands of the bipolar plateare etched back;

FIG. 11 is a plan view of a system for depositing the various layers onthe bipolar plates according to various embodiments of the invention;and

FIG. 12 illustrates one embodiment of the invention including a plasmaassisted chemical vapor deposition reaction chamber useful in a process;

FIG. 13 illustrates embodiments of the invention including a product anda process wherein the coating flows through the channels of a bipolarplate;

FIG. 14A illustrates a process according to one embodiment of theinvention wherein a bipolar plate is molded and includes a polymer richskin;

FIG. 14B illustrates process according to one embodiment of theinvention including depositing a coating over the skin of FIG. 14A;

FIG. 14C illustrates a process according to one embodiment of theinvention including removing the coating and skin over the lands of thebipolar plate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

One embodiment of the invention includes a fuel cell component having asubstrate, such as, but not limited to, a bipolar plate having a coatingincluding nanoparticles. The nanoparticles may have sizes ranging fromabout 2 to about 100 nm; preferably, about 2 to about 20 nm; and mostpreferably about 2 to about 5 nm. The nanoparticles may includeinorganic and/or organic materials. The coating may also include acompound including hydroxyl, halide, carboxyl, ketonic and/or aldehydefunctional groups. The coating makes the fuel cell component, such as abipolar plate, hydrophilic.

One embodiment of the invention includes a fuel cell component having ahydrophilic coating comprising nanoparticles having hydrophilic sidechains.

One embodiment of the invention includes a fuel cell component having ahydrophilic coating comprising nanoparticles comprising 10 to 90 weightpercent inorganic structures, 5 to 70 weigh percent hydrophilic sidechains, and 0 to 50 weight percent organic side chains having functionalgroups. Such a combination of materials provides hydrophilization ofsurfaces, wherein the coating consists of nano-scale particles havinghydrophilic side chains, which can be sintered.

In one embodiment of the invention the hydrophilic side chains areamino, sulfonate, sulfate, sulfite, sulfonamide, sulfoxide, carboxylate,polyol, polyether, phosphate, or phosphonate groups.

In one embodiment of the invention the coating may include organic sidechains and wherein the functional groups of the organic side chains areepoxy, acryloxy, methacryloxy, glycidyloxy, allyl, vinyl, carboxyl,mercapto, hydroxyl, amide or amino, isocyano, hydroxy, or silanolgroups. In one embodiment of the invention the coating has pH rangingfrom 3 to 10.

Another embodiment of the invention includes depositing a slurrysolution on a fuel cell component. The slurry solution includesnanoparticles and a vehicle, and thereafter driving off the vehicle. Thevehicle may include water, alcohol, other suitable solvents, orcombinations thereof. In one embodiment the slurry includes 4-5 weightpercent nanoparticles with the remaining portion being the vehicle. Inone embodiment the vehicle may be driven off at a temperature rangingfrom about 80 to about 180° C. The curing period may range from 10minutes at 80° C. to 10 seconds at 180° C.

Another means of applying nanoparticles to render a surface hydrophilicis the sol-gel process which involves the transition of a system from acolloidal state into a solid gel. The colloid is made of solidnanoparticles suspended in a liquid phase. In a typical sol-gel process,the colloidal suspension is first formulated, from which thenanoparticles particles condense in a new phase, the gel, in which asolid macromolecule is immersed in a solvent. Through subsequent dryingand curing, according to the same temperature ranges stated above, thefinal material constitutes a relatively continuous hydrophilic coating.

Suitable slurry materials are available from Nano-X GmbH under thetradenames HP 3408 and HP 4014. The slurry materials provide hydrophiliccoatings capable of surviving fuel cell operating condition for morethan 2500 hours. The coating may be formed on metals, such as aluminumand high-grade stainless steel, polymeric substrates, and electricallyconductive composite substrates, such as bipolar plates.

US patent application number 2004/0237833, the disclosure of which ishereby incorporated by reference, describes a number of ways to make aslurry useful in the present invention which are duplicated hereafter.

Example 1

221.29 g (1 mol) 3-aminopropyl triethoxy silane are added to 444.57 gsulfosuccinic acid, while stirring, and heated to 120° C. in a siliconebath for 5 h. After the reaction mixture has cooled, 20 g of the viscousfluid are mixed with 80 g (0.38 mol) tetraethoxy silane, and absorbed in100 g ethyl alcohol. The solution is then mixed with 13.68 g (0.76 mol)of a 0.1 N HCl solution, and tempered in a water bath overnight, at 40°C. This results in hydrophilic nanoparticles having reactive end groupsof approximately 2 nm. The resulting solution is diluted with a mixtureof 1/3 water and 2/3 N-methyl pyrrolidone (NMP), to a solid substancecontent of 5%, and applied to a glass plate by spraying, in a wet filmthickness of 10 to 20 .μm. Subsequently, the substrate is compacted in acirculating air drying cabinet for 3 h, at 150° C.

Example 2

221.29 g (1 mol) 3-aminopropyl triethoxy silane are added to 444.57 gsulfosuccinic acid, while stirring. Then the solution is heated to 130°C. in a silicone bath. After a reaction time of 1 h, 332.93 g of analkaline-stabilized aqueous silica gel solution of the Levasil 300/30%type (pH=10) are added to the reaction solution, while stirring. After areaction time of 12 h, the mixture is diluted with water, to a solidsubstance content of 5%. This results in hydrophilic nanoparticleshaving reactive end groups of approximately 15 nm. The system is appliedto plasma-activated polycarbonate sheets by means of inundation, andsubsequently dried in a circulating air drying cabinet for 5 h, at 130°C.

Example 3

123.68 g (0.5 mol) 3-isocyanatopropyl triethoxy silane are added to 600g (1 mol) polyethylene glycol 600, and heated to 130° C. in a siliconebath, after adding 0.12 g dibutyl tin laurate (0.1 wt.-% with regard to3-isocyanatopropyl triethoxy silane). 25 g (0.12 mol) tetraethoxy silaneand 33.4 g (0.12 mol) 3-glycidyloxy propyl triethoxy silane are added to50 g of the resulting solution (solution A), while stirring. Afteradding 15.12 g (0.84 mol) of a 0.1 N HCl solution, the mixture ishydrolyzed and condensed at room temperature for 24 h. This results inhydrophilic nanoparticles having reactive end groups of approximately 5nm.

Example 4

12.5 g (0.05 mol) 3-methacryloxy propyl trimethyloxy silane, 12.5 g of a20% aqueous CeO₂ solution (from Aldrich), and 50 g ethyl alcohol areadded to 50 g of the solution A described in Exemplary Embodiment 3,while stirring, in order to homogenize the mixture, and hydrophilizationtakes place for 48 h. After adding 0.375 g Ingacure 184 from CibaSpezialitaten Chemie (3 wt.-% with reference to 3-methacryloxy propyltrimethoxy silane), the mixture is applied to a flamed polycarbonatesheet by means of spraying, in a wet film thickness of at most 30 μm,and first dried thermally in a circulating air drying cabinet at 130° C.for 10 min. This is followed by photochemical drying with Hg emittershaving a radiation output of 1-2 J/cm².

The Fourier-transform infrared (FTIR) spectra illustrated in FIGS. 1 and2 are for two different coatings including nanoparticles according toone embodiment of the invention. In both FIG. 1 and FIG. 2, a small“shoulder” peak appears on the left side of the main Si—O—Si peak,representing a relatively low Si—R content where R is a saturated orunsaturated carbon chain including at least one carbon atom.

A scanning electron micrograph of the preferred coating is illustratedin FIG. 3. The coating is porous and the nanoparticles have a noduleshape.

The scope of the invention is not limited to the above describematerials and methods of making the same, but includes other coatingsincluding nanoparticles formed on a fuel cell component. The followingis a description of additional embodiments of coatings and methods ofmaking the same.

For example, suitable nanoparticles include but are not limited to SiO₂,other metal oxides, such as HfO₂, ZrO₂, Al₂O₃, SnO₂, Ta₂O₃, Nb₂O₅, MoO₂,IrO₂, RuO₂, metastable oxynitrides, nonstoichiometric metal oxides,oxynitrides, and derivatives thereof including carbon chains orincluding carbon, and mixtures thereof.

In one embodiment the coating is hydrophilic and includes at least oneSi—O group, at least one polar group and at least one group including asaturated or unsaturated carbon chain. In one embodiment of theinvention the polar group may include a hydroxyl, halide, carboxyl,ketonic or aldehyde functional groups. In one embodiment of theinvention the carbon chain may be saturated or unsaturated and may havebetween 1 and 4 carbon atoms. The coating may have additional elementsor compounds, including, for example, Au, Ag, Ru, Rh, Pd, Re, Os, Ir,Pt, rare earth metals, alloys thereof, polymeric carbon or graphite toimprove conductivity.

In one embodiment of the invention the coating includes an Si—O groupand an Si—R group wherein R includes a saturated or unsaturated carbonchain, and wherein the molar ratio of Si—R groups to Si—O groups rangesfrom 1/8 to 1/2, preferably 1/4 to 1/2. In another embodiment of theinvention the coating further includes hydroxyl groups to improve thehydrophilicity of the coating.

Another embodiment of the invention includes a fuel cell componenthaving a component with a coating thereon, and wherein the coating isderived from a siloxane. The siloxane may be linear, branched or cyclic.In one embodiment the siloxane has the formula R₂SiO, wherein R is analkyl group.

Another embodiment of the invention includes a fuel cell componenthaving a component with a coating thereon, and wherein the coating isderived from a material having the formula

wherein R₁, R₂, R₃, R₄, R₅, and R₆, each may be H, O, Cl, or a saturatedor unsaturated carbon chain having 1 to 4 carbon atoms, and wherein R₁,R₂, R₃, R₄, R₅, and R₆, may be the same or different.

Another embodiment of the invention includes a product comprising a fuelcell component having a coating formed thereon and wherein the coatingis formed by a process comprising plasma enhanced chemical vapordeposition of the coating from a precursor gas comprising a materialhaving the formula

wherein R₁, R₂, R₃, R₄, R₅, and R₆, each may be H, O, Cl, or a saturatedor unsaturated carbon chain having 1 to 4 carbon atoms, and wherein R₁,R₂, R₃, R₄, R₅, and R₆, may be the same or different, and furthercomprising treating the plasma enhanced chemical deposition depositedcoating to provide polar groups. In another embodiment of the invention,at least one of R₁, R₂, R₃, R₄, R₅, or R₆ is a carbon chain with atleast one carbon atom.

Another embodiment of the invention includes post-treating the depositedcoating comprising subjecting the coating to a plasma comprising oxygento produce polar groups in the deposited coating.

Another embodiment of the invention includes a fuel cell componenthaving a coating thereon, wherein the coating includes nanoparticleshaving a size ranging from 1 to 100 nm, preferably 1 to 50, and mostpreferably 1 to 10 nm, and wherein the nanoparticles comprise a compoundcomprising silicon, a saturated or unsaturated carbon chain and a polargroup.

Another embodiment of the invention includes a fuel cell having abipolar plate with a hydrophilic coating thereon, and a diffusion mediapositioned adjacent the bipolar plate. The hydrophilic coating includesnodule shaped nanoparticles and the coating is sufficiently porous sothat fibers from a diffusion media position adjacent the coating on thebipolar plate extend through the coating to provide an electrical paththrough the coating from the bipolar plate to the diffusion media.

Hydrophilic coating may also be required on the anode side of thebipolar plates, because anode water accumulation is known to influenceoperational stability under some conditions, and is suspected to impactelectrode and membrane durability through hydrogen starvation. However,it is expected that the rate of coating dissolution on the anode sidewill be faster than on the cathode side as a result of higher HFconcentration in the water produced during fuel cell operation.Therefore, the anode coating should be thicker than the cathode coatingto achieve durability to the end of fuel cell life. In one embodiment ofthe invention, the mean anode coating thickness is approximately 15%greater than the mean cathode coating thickness.

The coating should be thick enough to accommodate the rate of materialdissolution in the dilute HF environment within the fuel cell toend-of-life. Conversely, the coating should be thin enough, with thepreferred discontinuous morphology, to minimize the added electricalresistance. In one embodiment the coating may have a mean thickness of80-100 nm.

In one embodiment of the invention, the precursor gas is preferablyhexamethyl disiloxane (HMDSO) but can be selected from among inorganicor organic derivatives from siloxanes, silanols or silane basedchemistry, or other carbon and/or silicon containing gases and/orliquids. In one embodiment of the invention, the coating processinvolves micro-wave plasma-enhanced chemical vapor deposition (CVD)using hexamethyl disiloxane (HMDSO) precursor and pure oxygen as acarrier gas, which results in a siloxane-like (SiOx) coating. Themicrowave frequency is fixed at 2.45 GHz. The process temperature may berelatively low, in the range of ambient to 45° C., so that any practicalbipolar plate material can be coated without concerns of thermal damageor distortion. The actual application of the hydrophilic coatingmaterial, and its resulting chemical and physical structure, iscontrolled by the six adjustable process parameters associated with thecoating apparatus, in this case the PLASMAtech Model 110: operated at apressure ranging from 0 to 500 Pa; preferably: 10 to 100 Pa; and mostpreferably: 30 Pa.; at a microwave power ranging from 50 W to 10 kW,preferably: 100 W to 1 kW, and most preferably: 200 to 300 W for a CVDreactor with volume of 110 liters. The precursor gas is preferablyhexamethyl disiloxane (HMDSO) but can be selected from among inorganicor organic derivatives from siloxanes, silanols or silane basedchemistry, or other carbon and/or silicon containing gases and/orliquids as described above. The carrier gas is preferably oxygen, butmay include at least one of nitrogen, nitrous oxide, argon,argon-oxygen, or their mixtures, or mixtures with other gases inappropriate ratios.

The ratio of precursor-to-carrier gas volumetric flow rate has asignificant effect on the resulting chemical structure and morphology ofthe coated layer. In general, particularly with a siloxane-containingprecursor, a small precursor-to-carrier ratio will result in a densercoating that approaches the chemical structure of pure SiO₂. As thisratio is increased, the organic content of the coating increases, whichmost likely reduces the hydrophilicity (i.e., increases static contactangle) but also increases the porosity of the coating structure. It isthe balance of these characteristics that is critical for application ina fuel cell, to attain the required contact angle, while also minimizingthe added electrical resistance. In one embodiment of the invention, theprecursor-to-carrier flow ratio is 2 to 16%, preferably: 4 to 12%, andmost preferably: 8 to 10%.

The absolute gas flow rates will be functions of the total reactorvolume. For the PLASMAtech Model 110 used to produce the bipolar platecoatings described herein, the gas flow ranges (assuming a gas flowratio of 8 to 10% as discussed above) are as follows: Applicable ranges:Precursor=2-50 ml/min; Carrier=20-625 ml/min, preferably:Precursor=10-30 ml/min; Carrier=100-375 ml/min, and most preferably:Precursor=15-20 ml/min; Carrier=150-250 ml/min.

The reactor time will dictate the thickness of the coated layer, but mayalso impact the coating morphology. The time may be selected to producea coating that is thick enough to accommodate the rate of materialdissolution in the dilute HF environment within the fuel cell toend-of-life. Conversely, the coating should be thin enough, with thepreferred discontinuous morphology, to minimize the added electricalresistance. This combination of coating characteristics was optimized byusing a reactor time of 4 minutes per side of the bipolar plates, toproduce a coating with mean thickness of 80-100 nm.

A post-treatment process may be required to introduce polar functionalmoieties (predominantly hydroxyl groups) onto the base SiO_(x)structure, thereby further enhancing the material hydrophilicity. In oneembodiment of the invention, this is done by exposing the SiO_(x) filmsto a reactive oxygen plasma which would activate the SiO_(x) coating bybreaking organic bonds and forming hydroxyl, carboxyl and aldehydefunctional groups. This activation by post-treatment also enhances thematerial porosity, which may further reduce the electrical resistance.In another embodiment, the coating is reacted with a chemical to producethe polar groups. In another embodiment, the polar groups are introducedby applying a thin layer of a hydrophilic coating.

In one embodiment of the invention, the post-treatment process involvesexposure to a microwave-generated oxygen plasma environment for 0 to 5minutes, preferably: 0.5 to 3 minutes, and most preferably: 1.5 minutes.

FIG. 4 is a cross-sectional view of a fuel cell 10 that is part of afuel stack of the type discussed above. The fuel cell 10 includes acathode side 12 and an anode side 14 separated by an electrolytemembrane 16. A cathode side diffusion media layer 20 is provided on thecathode side 12, and a cathode side catalyst layer 22 is providedbetween the membrane 16 and the diffusion media layer 20. Likewise, ananode side diffusion media layer 24 is provided on the anode side 14,and an anode side catalyst layer 26 is provided between the membrane 16and the diffusion media layer 24. The catalyst layers 22 and 26 and themembrane 16 define an MEA. The diffusion media layers 20 and 24 areporous layers that provide for input gas transport to and watertransport from the MEA. Various techniques are known in the art fordepositing the catalyst layers 22 and 26 on the diffusion media layers20 and 24, respectively, or on the membrane 16.

A cathode side flow field plate or bipolar plate 18 is provided on thecathode side 12 and an anode side flow field plate or bipolar plate 30is provided on the anode side 14. The bipolar plates 18 and 30 areprovided between the fuel cells in the fuel cell stack. A hydrogenreactant gas flow from flow channels 28 in the bipolar plate 30 reactswith the catalyst layer 26 to dissociate the hydrogen ions and theelectrons. Airflow from flow channels 32 in the bipolar plate 18 reactswith the catalyst layer 22. The hydrogen ions are able to propagatethrough the membrane 16 where they electro-chemically react with theoxygen in the airflow and the electrons in the catalyst layer 22 togenerate water as a by-product.

In this non-limiting embodiment, the bipolar plate 18 may include twosheets 34 and 36 that are stamped and welded together. The sheet 36defines the flow channels 32 and the sheet 34 defines flow channels 38for the anode side of an adjacent fuel cell to the fuel cell 10. Coolingfluid flow channels 40 are provided between the sheets 34 and 36, asshown. Likewise, the bipolar plate 30 includes a sheet 42 defining theflow channels 28, a sheet 44 defining flow channels 46 for the cathodeside of an adjacent fuel cell, and cooling fluid flow channels 48. Inthe embodiments discussed herein, the sheets 34, 36, 42 and 44 are madeof an electrically conductive material, such as stainless steel,titanium, aluminum, polymeric carbon composites, etc.

According to one embodiment of the invention, the bipolar plates 18 and30 include a coating 50 that makes the plates 18 and 30 hydrophilic. Thehydrophilicity of the coating 50 causes the water within the flowchannels 28 and 32 to form a film instead of water droplets so that thewater does not significantly block the flow channels. Particularly, thehydrophilicity of the coating 50 decreases the contact angle of wateraccumulating within the flow channels 32, 38, 28 and 46, preferablybelow 40°, so that the reactant gas is still able to flow through thechannels 28 and 32 at low loads. In one embodiment, the coating 50 is athin film, for example, in the range of 5 to 50 nm, so that theconductivity of the sheets 34, 36, 42 and 44 still allows electricity tobe effectively coupled out of the fuel cell 10.

According to another embodiment of the present invention, the coating 50is combined with a conductive material, such as Au, Ag, Ru, Rh, Pd, Re,Os, Ir, Pt, rare earth metals, alloys thereof, polymeric carbon orgraphite, that increases the conductivity of the coating 50. By makingthe bipolar plates 18 and 30 more conductive, the electrical contactresistance and the ohmic losses in the fuel cell 10 are reduced, thusincreasing cell efficiency. Also, a reduction in compression force inthe stack can be provided, addressing certain durability issues withinthe stack.

Before the coating 50 is deposited on the bipolar plates 18 and 30, thebipolar plates 18 and 30 may be cleaned by a suitable process, such asion beam sputtering, to remove the resistive oxide film on the outsideof the plates 18 and 30 that may have formed. The coating can bedeposited on the bipolar plates 18 and 30 by any suitable techniqueincluding, but not limited to, physical vapor deposition processes,chemical vapor deposition (CVD) processes, thermal spraying processes,sol-gel, spraying, dipping, brushing, spinning on, or screen printing.Suitable examples of physical vapor deposition processes includeelectron beam evaporation, magnetron sputtering and pulsed plasmaprocesses. Suitable chemical vapor deposition processes include plasmaenhanced CVD and atomic layer deposition processes. CVD depositionprocesses may be more suitable for the thin film layers of the coating50.

FIG. 5 is a broken-away, cross-sectional view of a bipolar plate 60including reactant gas flow channels 62 and lands 64 therebetween,according to another embodiment of the invention. The bipolar plate 60is applicable to replace the bipolar plate 18 or 30 in the fuel cell 10.In this embodiment, a coating 50 is deposited as random islands 68 onthe plate 60 so that the conductive material of plate 60 is exposed atareas 70 between the islands 68. The coating islands 68 provide thedesired hydrophilicity of the plate 60, and the exposed areas 70 providethe desired conductivity of the plate 60. In this embodiment, theislands 68 may best be deposited by a physical vapor deposition process,such as electron beam evaporation, magnetron sputtering and pulsedplasma processes. In one embodiment, the islands 68 are deposited to athickness between 50 and 100 nm.

FIG. 6 is a broken-away, cross sectional view of a bipolar plate 72including reactant gas flow channels 74 and lands 76 therebetween,according to another embodiment of the invention. In this embodiment, acoating 78 is deposited on the bipolar plate 72. The coating 78 is thenremoved over the lands 76 by any suitable process, such as polishing orgrinding, to expose the conductive material of the plate 72 at the lands76. Therefore, the flow channels 74 include the hydrophilic coating, andthe lands 76 are conductive so that electricity is conducted out of afuel cell. In this embodiment, the coating 78 can be deposited thickerthan the embodiments discussed above, such as 100 nm to fpm, because theplate 72 can be less conductive in the channels 74.

FIG. 7 is broken-away, cross-sectional view of a bipolar plate 82including reactant gas flow channels 74 and lands 76, according toanother embodiment of the present invention. In this embodiment, thebipolar plate 82 has an electrically conductive protective layer 52thereon. A coating 78 according to the present invention is providedoverlying only the channels 74 of the bipolar plate 82.

FIG. 8 illustrates one embodiment of a process according to the presentinvention including first selectively forming a mask 200 over the lands76 of a bipolar plate 18 and thereafter depositing a coating 50, whichmay include silicon, over the bipolar plate 18 including the mask 200.The mask 200 may be a hard physical mask, a viscous liquid or gel likematerial, or a removable material such as a photoresist. As shown inFIG. 9, the mask 200 over the lands 76 is removed to leave the coating50 only overlying the channel 74 of the bipolar plate 18.

FIG. 10 illustrates one embodiment of a process according to the presentinvention including first depositing a coating 50 including silicon overthe bipolar plate 18 including the lands 76 and channels 74, thenselectively forming a mask 200, such as a photoresist or water solublematerial, over the channels 74 of a bipolar plate, and thereafter thecoating 50 over the lands 76 of the bipolar plate are etched back. Theetch may be accomplished using a wet or dry etch process, provided theetching does not damage the bipolar plate 18. In one embodiment thecoating 50 over the lands 76 may be remove by an argon plasma andthereafter any remaining portion of the mask is removed.

FIG. 11 is a plan view of a system 100 for depositing the various layerson the bipolar plates discussed above. The system 100 is intended torepresent any of the techniques mentioned above, including, but notlimited to, blasting, physical vapor deposition processes, chemicalvapor deposition processes, thermal spraying processes and sol-gel. Inthe system 100, an electron gun 102 heats a material 104 that causes thematerial 104 to be vaporized and deposited on a substrate 106,representing the bipolar plate, to form a coating 108 thereon. Inanother process, the system 100 includes an ion gun 110 that directs abeam of ions to a sputtering surface 112 that releases material, such asa metal oxide, to deposit the coating 108. In another embodiment, thecoating 50 may be applied by spraying, dipping, brushing, spinning on,or screen printing.

FIG. 12 illustrates one embodiment of a plasma assisted chemical vapordeposition reactor 400 useful in a process according to one embodimentof the present invention. The reactor 400 includes a plurality of walls402 and a ceiling 404. A plurality of gas charging ports 406, 408, 410may be provided through the walls 402 or ceiling 404 for chargingreaction and carrier gases into the reactor chamber 412. A liquidcharging dispenser 414 may also be provided. The reactor may include amicrowave generating means 416 and an Rf generating means 418 to producea plasma in the reactor chamber 412. A chuck 420 may be provided tosupport a fuel cell component such as a bipolar plate.

FIG. 13 illustrates one embodiment of the invention including a productand a process wherein the slurry according to the present inventionflows through the channels 74 of a bipolar plate 18, for example bypushing the slurry through headers (not shown) of the bipolar plate 18.Thereafter, the vehicle is driven off at a temperature ranging from 80to 180° C. to leave a permanent coating 50 overlying a portion of thechannel 74.

FIG. 14A illustrates a process according to one embodiment of thepresent invention wherein a bipolar plate 18 is molded and includes apolymer rich skin 500 over the outer surface, including the lands 76 andchannels 74.

FIG. 14B illustrates a process according to one embodiment of theinvention including depositing a coating over the skin 500 of FIG. 14A,wherein the coating 50 also overlies the lands 76 and channels 74 of thebipolar plate.

FIG. 14C illustrates a process according to one embodiment of theinvention including removing the coating 50 and skin 500 over the lands76 of the bipolar plate 18.

In another embodiment of the invention a coating having Si—O and Si—R(where R is a saturated or unsaturated carbon chain) groups areselectively deposited on a flat substrate such as a foil of stainlesssteel and there after formed, for example by stamping, into a bipolarplate having a gas flow field including a plurality of lands andchannels, and wherein the coating is deposited in the channels.

In another embodiment of the invention, a coating having Si—O and Si—R(where R is a saturated or unsaturated carbon chain) groups may beformed on a substrate using a variety of chemistries including materialcomprising Si and materials including carbon. For example, the coatingmay be produced using plasma assisted CVD with silane (SiH₄), oxygen anda carbon based gas or liquid. In another embodiment, the coating may beproduced using plasma assisted CVD with TEOS which istetraethyloxysilate or tetraethoxysilane (Si(C₂H₅O)₄), or MTEOS which ismethyltriethoxysilane, and oxygen or ozone, and optionally a carbonbased gas or liquid.

When the terms “over”, “overlying”, “overlies” or the like are usedherein with respect to the relative position of layers to each othersuch shall mean that the layers are in direct contact with each other orthat another layer or layers may be interposed between the layers.

The description of the invention is merely exemplary in nature and,thus, variations thereof are not to be regarded as a departure from thespirit and scope of the invention.

1. A product comprising a fuel cell component comprising a substrate anda coating overlying the substrate, the coating comprising nanoparticleshaving sizes ranging from 2 to 100 nm.
 2. A product as set forth inclaim 1 wherein the coating is hydrophilic.
 3. A product comprising afuel cell bipolar plate comprising a hydrophilic coating overlying thebipolar plate, the coating comprising nanoparticles having a sizesranging from about 2 to about 100 nm.
 4. A product as set forth in claim3 wherein the nanoparticles have a size ranging from about 2 to about 20nm.
 5. A product as set forth in claim 3 wherein the nanoparticles havea size ranging from about 2 to about 5 nm.
 6. A product as set forth inclaim 3 wherein the nanoparticles comprise at least one of SiO₂, HfO₂,ZrO₂, Al₂O₃, SnO₂, Ta₂O₅, Nb₂O₅, MoO₂, IrO₂, RuO₂, metastableoxynitrides, nonstoichiometric metal oxides, oxynitrides, or derivativesthereof.
 7. A product as set forth in claim 3 wherein the nanoparticlesinclude at least one of hydroxyl, halide, carboxyl, ketonic or aldehydefunctional groups.
 8. A product as set forth in claim 3 wherein thecoating comprises 10 to 90% inorganic structures, and 5 to 70%hydrophilic side chains.
 9. A product as set forth in claim 3 whereinthe coating is prepared by a so-gel process.
 10. A product as set forthin claim 7 further comprising organic side chains having functionalgroups.
 11. A product as set forth in claim 8 wherein the inorganicstructures comprises silicon dioxide or nonstoichiometric silicon oxide.12. A product as set forth in claim 8 wherein the inorganic structurescomprise zirconium oxide.
 13. A product as set forth in claim 8 whereinthe hydrophilic side chains include at least one of amino, sulfonate,sulfate, sulfite, sulfonamide, sulfoxide, carboxylate, polyol,polyether, phosphate, or phosphonate groups.
 14. A product as set forthin claim 8 wherein the functional groups comprises at least one ofepoxy, acryloxy, methacryloxy, glycidyloxy, allyl, vinyl, carboxyl,mercapto, hydroxyl, amide or amino, isocyano, hydroxy, or silanolgroups.
 15. A product as set forth in claim 3 wherein the nanoparticleshave a nodule shape.
 16. A product comprising: a fuel cell componentcomprising a substrate and a first coating overlying the substrate, thecoating comprising nanoparticles each having a size ranging from about 2to 100 nm, the nanoparticles comprising at least one Si—O group, atleast one polar group and at least one group including a saturated orunsaturated carbon chain.
 17. A product as set forth in claim 16 whereinthe polar group comprises a hydroxyl group, halide, carboxyl, ketonic,or aldehyde functional group.
 18. A product as set forth in claim 16wherein the carbon chain has between 1 and 4 carbon atoms.
 19. A productas set forth in claim 16 wherein the coating further comprises anelectrically conductive material.
 20. A product as set forth in claim 19wherein the electrically conductive material comprises at least one ofAu, Ag, Ru, Rh, Pd, Re, Os, Ir, Pt, rare earth metals, alloys thereof,polymeric carbon or graphite.
 21. A product as set forth in claim 16wherein the component comprises a bipolar plate.
 22. A product as setforth in claim 16 wherein the substrate comprises a metal.
 23. A productas set forth in claim 22 wherein the substrate comprises a polymericcomposite material comprising fiber structures for increased stiffnessand/or conductivity.
 24. A product as set forth in claim 22 furthercomprising a second coating comprising an electrically conductivematerial and wherein the second coating overlies the substrate and thefirst coating overlies the second coating.
 25. A product as set forth inclaim 16 wherein the carbon chain is linear, branched or cyclic.
 26. Aproduct comprising: a fuel cell component comprising a substrate and afirst coating overlying the substrate, the coating comprisingnanoparticles each having a size ranging from about 2 to 100 nm, thenanoparticles comprising a compound comprising at least one Si—O groupand an Si—R group wherein R includes a saturated or unsaturated carbonchain, and wherein the molar ratio of Si—R groups to Si—O groups rangesfrom 1/8 to 1/2.
 27. A product as set forth in claim 26 wherein thecarbon chain has 1 to 4 carbon atoms.
 28. A product as set forth inclaim 26 wherein the first coating further comprises polar groups toimprove the hydrophilicity of the coating.
 29. A product as set forth inclaim 28 wherein the polar groups comprises hydroxyl groups.
 30. Aproduct as set forth in claim 26 wherein the coating further comprisesan electrically conductive material.
 31. A product as set forth in claim30 wherein the electrically conductive material comprises at least oneof Au, Ag, Ru, Rh, Pd, Re, Os, Ir, Pt, rare earth metals, alloysthereof, polymeric carbon or graphite.
 32. A product as set forth inclaim 26 wherein the component comprises a bipolar plate.
 33. A processcomprising depositing a slurry over at least a portion of a fuel cellcomponent comprising a substrate, the slurry comprising nanoparticleshaving sizes ranging from 2 to 100 nm, and a vehicle; and driving offthe vehicle to leave a coating comprising nanoparticles having sizesranging from 2 to 100 nm.
 34. A process as set forth in claim 33 whereinthe coating is hydrophilic.
 35. A process comprising depositing a slurryover at least a portion of a fuel cell bipolar plate, the slurrycomprising hydrophilic nanoparticles having a sizes ranging from 2 to100 nm and a hydrophilic side chain covalently attached to thenanoparticles, and a vehicle; and driving off the vehicle to leave ahydrophilic coating comprising nanoparticles having sizes ranging from 2to 100 nm.
 36. A process as set forth in claim 35 wherein thenanoparticles have sizes ranging from about 2 to about 20 nm.
 37. Aprocess as set forth in claim 35 wherein the nanoparticles have sizesranging from about 2 to about 5 nm.
 38. A process as set forth in claim35 wherein the nanoparticles comprises at least one of SiO₂, HfO₂, ZrO₂,Al₂O₃, SnO₂, Ta₂O₅, Nb₂O₅. MoO₂, IrO₂, RuO₂, metastable oxynitrides,nonstoichiometric metal oxides, oxynitrides, or derivatives thereof. 39.A process as set forth in claim 35 wherein the nanoparticles include atleast one of hydroxyl, halide, carboxyl, ketonic or aldehyde functionalgroups.
 40. A process as set forth in claim 35 wherein the coatingcomprises 10 to 90% inorganic structures, and 5 to 70% hydrophilic sidechains.
 41. A process as set forth in claim 39 wherein the nanoparticlesfurther comprise organic side chains having functional groups.
 42. Aprocess as set forth in claim 40 wherein the inorganic structures aresilicon dioxide or nonstoichiometric silicon oxide.
 43. A process as setforth in claim 40 wherein the inorganic structures comprise zirconiumoxide.
 44. A process as set forth in claim 39 wherein the hydrophilicside chains include at least one of amino, sulfonate, sulfate, sulfite,sulfonamide, sulfoxide, carboxylate, polyol, polyether, phosphate, orphosphonate groups.
 45. A process as set forth in claim 39 wherein thefunctional groups comprise at least one of epoxy, acryloxy,methacryloxy, glycidyloxy, allyl, vinyl, carboxyl, mercapto, hydroxyl,amide or amino, isocyano, hydroxy, or silanol groups.
 46. A process asset forth in claim 35 wherein the nanoparticles have a nodule shape. 47.A process as set forth in claim 35 wherein the bipolar plate includes agas flow field including a plurality of lands and channels, and whereinthe coating is over the lands and channels.
 48. A process as set forthin claim 47 further comprising removing the coating from the lands. 49.A process as set forth in claim 47 wherein the slurry is deposited onlyover the channels.
 50. A process as set forth in claim 47 wherein theslurry is deposited over only a portion of the channel.
 51. A process asset forth in claim 47 wherein the depositing of a slurry over at least aportion of a fuel cell bipolar plate comprises flowing the slurrythrough the flow field.
 52. A process as set forth in claim 47 furthercomprising depositing a mask over a portion of the bipolar plate priorto depositing the slurry, and wherein the mask leaves portions of thebipolar plate exposed.
 53. A process as set forth in claim 52 whereinthe slurry is deposited over the mask and exposed portions of thebipolar plate.
 54. A process as set forth in claim 53 further comprisingremoving the mask and the coating over the mask.
 55. A process as setforth in claim 52 wherein the mask comprises one of a hard mask,photoresist mask, or water washable mask.
 56. A process as set forth inclaim 55 wherein the mask is over the lands.
 57. A process as set forthin claim 48 and further comprising depositing a mask over the coatingthat is over the channels and removing the coating over the lands, andthereafter removing any remaining portion of the mask.
 58. A processcomprising forming a coating over a bipolar plate, wherein the bipolarplate includes a flow field including a plurality of lands and channels,comprising flowing a liquid coating through the channels only of thebipolar plate and thereafter curing the liquid coating.
 59. A process asset forth in claim 58 wherein the coating comprises nanoparticles and avehicle.
 60. A process as set forth in claim 59 wherein the curing theliquid coating comprises driving off the vehicle to provide a curedcoating.
 61. A process as set forth in claim 60 wherein the curedcoating is hydrophilic.
 62. A process comprising depositing a slurry onat least a portion of a fuel cell bipolar plate, the slurry comprisinghydrophile nanoparticles having a sizes ranging from 2 to 100 nm and ahydrophilic side chain covalently attached to the nanoparticles, and avehicle; and driving off the vehicle to leave a hydrophilic coatingcomprising nanoparticles having sizes ranging from 2 to 100 nm on theportion of the fuel cell bipolar plate.